There are various reasons for this sort of thing, and some are more easily explained than others.
There are some benefits purely from an operational view. For example, it's a useful contingency measure to send the longer train on the shorter journey because it gives more options for subsequent stock use. If the 8 coach train is heading all the way to Portsmouth, you have less time and fewer opportunities to split the train, balance stock and send it elsewhere along the coast. This is also a consideration on the Victoria-Ore services - it takes so long for the units to get to Ore that it can seriously muck up the stock allocations later in the day if anything needs to be changed. I've worked with Southern Control on this one in the fairly recent past and there's been a lot of head-scratching at times about how to retrieve a train which is merrily enjoying a round trip of several hours away from where it actually needs to be. This is why you get stock swaps or portions detached at places like Eastbourne. Unfortunately the West Coastway and routes via Barnham are somewhat less flexible in this regard until you reach the termini - attempting extra manoeuvres at multi-directional hubs like Barnham can be a risky business if you are tight for crew or pathing, so you have to wait until the terminus with more space and more capability.
Special events, popularity of destinations, ticket prices, etc. can all skew loadings but this is not always easy to tell. Sometimes it's seasonal, eg. links from the South Coast via Gatwick are usually at least slightly more popular around summertime, but other times the fluctuation with all the different special events, carnivals, festivals etc. is difficult to predict, given the diverse choices that passengers will make about how they'd like the destinations and large towns on Southern's network to link up for them.
Absolutely nothing to do with it. SDO has been operational on 377s for well over a decade and is frequently used for coastal services. The more minor ones typically have the shorter platforms (though not always) and passengers quickly adapt anyway. (This is often seen with commuter stations as well - for example, Ifield is fairly well-used, but only a maximum of 5 coaches can be accommodated - less than half of many train lengths calling there!)